This invention relates to a method of making a blank for a drill bit wherein the cutting edge is provided by an abrasive compact.
Abrasive compacts are well known in the art and are used extensively in industry for the abrading of various work pieces. They consist essentially of a mass of abrasive particles generally present in an amount of at least 70 percent, preferably 80 to 90 percent, by volume of the compact bonded into a hard conglomerate. Compacts are polycrystalline masses and can replace single large crystals in many applications. The abrasive particles of compacts are invariably ultra-hard abrasives such as diamond and cubic boron nitride.
Abrasive compacts generally contain a second phase or bonding matrix which contains a solvent/catalyst useful in synthesising the particles. In the case of cubic boron nitride, examples of suitable solvent/catalysts are aluminium or an alloy of aluminium with nickel, cobalt, iron, manganese or chromium. In the case of diamond, examples of suitable solvent/catalysts are metals of Group VIII of the Periodic Table such as cobalt, nickel or iron or an alloy containing such a metal.
As is known in the art, diamond and cubic boron nitride compacts are manufactured under conditions of temperature and pressure at which the abrasive particle is crystallographically stable.
Abrasive compacts may be bonded directly to a tool or shank for use. Alternatively, they may be bonded to a backing such as a cemented carbide backing prior to being mounted on a tool or shank. Such backed compacts are also known in the art as composite abrasive compacts. Bonding of the compact to the backing may be direct or through a bonding layer of a material different to the compact or backing.
Diamond abrasive compacts which contain a metallic phase are thermally sensitive and tend to degrade rather rapidly at temperatures above about 700.degree. C. at ambient pressure. Several diamond compacts are known which are thermally stable under these conditions. Such compacts are described, for example, in U.S. Pat. No. 4,224,380 and the complete specifications of South African Pat. Nos. 84/0053 and 85/2177.
In the manufacture of printed circuit boards it is often necessary to form a plurality of small holes therein. These holes are generally formed by means of a small diameter twist drill. Since such boards are abrasive, for example made of fibre reinforced plastic, it has been found that drill bits tipped with diamond abrasive compact are particularly suitable. Such drill bits are typically of fluted circular section. Examples of such drill bits and blanks for such drill bits can be found in European Patent Publication No. 0168953. Japanese Patent Publication No. 58/83790 describes hexagonal shaped composite abrasive compact inserts for drill bits. These inserts have a transverse dimension much greater than their thickness or depth dimension.